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I've asked a question that's mostly about optics with a little touch of thermodynamics to it: Can you use an invisibility shield to insulate from infrared radiation?

But I'm getting 2 close votes to close it as "not mainstream physics".

At worst I am definitely guilty of asking a borderline engineering question, and I wouldn't debate any close votes of that nature. But nothing in my question assumes anything other than classical optics.

Since I

  1. assume mirrors heat up when reflecting (which the linked question shows is widely believed to be true by the community, for reasons related to conservation of momentum + energy (which I believe is considered a mainstream theory)).

  2. Then question what happens if you consider an object like an invisibility cloak (which is definitely possible to build using sets of lenses). Here's a company that literally builds these things out of lenses for the military https://www.youtube.com/watch?v=KckHomYdiFc

It's just an extension of a thought experiment FROM mainstream physics, I even link a post that I build off of which is clearly actual mainstream physics as agreed by the community.

So why do people think that this is not physics? Is there any axiom or assumption I've made in the problem that is not widely believed to be true?

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  • $\begingroup$ Asking if magic allows you to violate physics is not mainstream. $\endgroup$ – Jon Custer Dec 16 '20 at 16:46
  • $\begingroup$ you can make objects disappear (in the visible spectrum) with lenses. Like invisibility cloak = "just a set of lenses" or if you want it to be more cloak like, then an object such as the following: youtube.com/watch?v=KckHomYdiFc would do $\endgroup$ – frogeyedpeas Dec 16 '20 at 16:47
  • $\begingroup$ Here's a tabletop demonstration of an invisibility cloak: youtube.com/watch?v=7W5T-qOQF50, I think there's been a misunderstanding on what I was assuming that led to the "magic" assumption by the closers. $\endgroup$ – frogeyedpeas Dec 16 '20 at 16:51
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    $\begingroup$ You can do various interesting things with, e.g. metamaterials. Such things do not eliminate blackbody radiation, they just shift it around to be non-ideal (not that many things are ideal in the first place). What you propose violates physics. $\endgroup$ – Jon Custer Dec 16 '20 at 16:53
  • $\begingroup$ I'm posing a question about trying to use (lens or metametarial, whatever invisibility strategy you prefer) to reduce the amount of blackbody radiation that an object is exposed to, from another distant object. I really don't feel convinced that this is not mainstream physics since the only argument thats been made so far is that I was assuming something physically impossible-- which I wasn't since it's already been realized experimentally for the visible spectrum. $\endgroup$ – frogeyedpeas Dec 16 '20 at 16:56
  • $\begingroup$ It is easy to reduce the amount of blackbody radiation (almost all materials are not ideal black bodies). What you can't do is eliminate it, and if there is any you will still come to thermodynamic equilibrium. $\endgroup$ – Jon Custer Dec 16 '20 at 17:00
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    $\begingroup$ The idea of an "invisibility cloak" is very unclear from your question. As I commented there, it sounds like you're literally just talking about something perfectly transparent across the entire EM spectrum. $\endgroup$ – JMac Dec 16 '20 at 17:13
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I just cast the 4th vote to close, so you're one away now. I'll tell you why I voted to close:

  1. Assuming that you could have made this "mainstream", then it would still fall under "engineering", as you noted, and I'd still vote to close. It's not worth a lot of time being fine about the reason.
  2. Your use of "invisibility" does not appear to have any physically relevant interpretation. You introduce the term here: "I.E. you need something like an invisibility cloak." with no real definition of what you mean by that. As it's written, it makes me think you mean like Harry Potter running around Hogwarts.
  3. The first part of your question "Is my intuition correct, and an IDEAL invisibility cloak around an object would be a perfect insulating material?" has no answer in physics as far as I can see, given how I understand your question. At best, it would go back to engineering and/or specifying enough detail to answer it.
  4. Probably part of the problem is that your question seems to be premised on your own unsupported assumption "The only way to beat this limit, would be to have a substance which absorbs and re-emits photons in the same direction that the photon came from (so now you have respected conservation of momentum and don't theoretically need to absorb heat)", which is probably not right and doesn't itself seem to be based in physics.

Could be that you could write a version of the underlying question differently and keep it mainstream, but what you actually wrote, on its face, does not seem mainstream and, anyway, probably would have been closed for the other possible reasons allude by me and by your original meta-question here. Making us work to ground your question in mainstream is attracting the close votes that you mentioned - or at least mine. Maybe others took a different view in coming to the same conclusion.

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  • $\begingroup$ your parts 3 and 4 are just re-statements of problem 2. So the issue here was really #2. (And you were definitely NOT the only person that thought this). I find it surprising that when I said "invisibility cloak" more people thought I meant harry potter, than the obvious physical interpretation of the term using lenses. I guess if I ask this in an engineering forum ill draw more pictures and make it clear what I mean. $\endgroup$ – frogeyedpeas Dec 16 '20 at 17:25
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    $\begingroup$ If there had been a physical interpretation that I thought was "obvious", I wouldn't have been thrown just by an informal description in context. I didn't (and still don't) see that interpretation in what's currently written. $\endgroup$ – Brick Dec 16 '20 at 17:46
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So I didn't vote to close for the non-mainstream reason, since it seemed like you were still trying to stay within the realm of real physics. However, I did vote to close for the "engineering reason" due to your second question

Is it possible in practice to build something which acts like an invisibility cloak for infrared, and has anyone done so?

But there are other issues with this. First, this isn't really a good question because an answer is just yes or no. This question isn't asking to understand any physics.

But another issue is that you are asking two different questions, which means the question also lacks focus.

So while I might not fully agree with the main closure reason, I still think the question should be closed in its current form.

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